Showing papers by "Illinois Institute of Technology published in 1987"

Oxygen ordering and the orthorhombic-to-tetragonal phase transition in YBa2Cu

Abstract: In situ neutron powder diffraction measurements show that the orthorhombic-to-tetragonal phase transition in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$, which occurs near 700\ifmmode^\circ\else\textdegree\fi{}C in a pure oxygen atmosphere, is an order-disorder transition in which the disordering of oxygen atoms into a normally vacant site destroys the one-dimensional Cu-O chains present in the room-temperature orthorhombic structure. For both structures, the oxygen stoichiometry decreases monotonically with increasing temperature. The transition temperature depends on the oxygen partial pressure and occurs when the stoichiometry is near Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.5}$. The tetragonal structure has a partially occupied, nearly octahedral Cu-O arrangement, in contrast to the orthorhombic structure which has one-dimensional Cu-O chains. The observed depression of the superconducting transition temperature in tetragonal $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$, which has been quenched from high temperature, could result either from the disordering of oxygen atoms which destroys the one-dimensional chains or from the absence of ${\mathrm{Cu}}^{3+}$ ions.

Iconic Indexing by 2-D Strings

Abstract: In this paper, we describe a new way of representing a symbolic picture by a two-dimensional string. A picture query can also be specified as a 2-D string. The problem of pictorial information retrieval then becomes a problem of 2-D subsequence matching. We present algorithms for encoding a symbolic picture into its 2-D string representation, reconstructing a picture from its 2-D string representation, and matching a 2-D string with another 2-D string. We also prove the necessary and sufficient conditions to characterize ambiguous pictures for reduced 2-D strings as well as normal 2-D strings. This approach thus allows an efficient and natural way to construct iconic indexes for pictures.

Incorporation of Pr in YBa2Cu3O7−δ: electronic effects on superconductivity

Abstract: Considerable excitement has been generated by the report of superconductivity above 90 K in a multi-phase Y–Ba–Cu oxide1. The superconducting phase was subsequently identified as YBa2Cu3O7−δ2,3. The complete replacement of Y by the trivalent rare-earths La–Lu, with the exception of Ce, Pr and Tb, yields a superconducting phase with a critical temperature (Tc) almost identical to that reported for the yttrium compound4–7. This is somewhat surprising as most of these ions carry significant magnetic moments, the presence of which rapidly decreases Tc in ordinary superconductors. The three exceptions noted, Ce, Pr and Tb, are the rare-earth ions which have a stable tetravalent state. Here we present the results of a study of the structure and transport properties of the series of compounds Y1−xPrxBa2Cu3O7−δ (x = 0−1.0), in which we observe a monotonic decrease in Tc and eventually a metal–nsulator transition with increasing Pr concentration. To date, this is the only rare-earth ion which has been found substantially to alter Tc upon partial substitution for Y.

Structure and crystal chemistry of the high-Tc superconductor YBa2Cu3O7−x

Abstract: Materials within the phase diagram Y2O3–BaO–CuO have recently generated great interest because of the observation of superconductivity with a critical temperature (Tc ) of ≳90 K1,2; the current consensus is that YBa2Cu3O7−x (x ≈ 0.2) is the superconducting phase (see, for example ref. 3). Hazen et al.4 concluded, from a single-crystal X-ray diffraction experiment, that its structure is based on the corner-linked octahedral perovskite structure: the tetragonal unit cell (space group P&4bar;m2) arises from a tripling of the c-axis resulting from the ordering of yttrium and barium cations and associated oxygen defects. Here we present high-resolution neutron powder diffraction data for this phase with x = 0.15(7). Our results agree with ref. 4 with respect to cation positions, but the location of one set of oxygen defects is substantially different and necessitates the lowering of lattice symmetry from tetragonal to orthorhombic (space group Pmmm), in agreement with separate findings5–8. In contrast to the 35-K superconductor, La1.85Ba0.15CuO4, the corner-linked CuO4 planar groups are connected not only as sheets in the a–b plane but also as chains parallel to the ft-axis. The average copper valence is 2.23 (5). From bond valence arguments we infer that the Cu2+ and Cu3+ ions preferentially occupy square pyramidal and square planar sites respectively. Crystal chemical considerations suggest that these structural features may be involved in the superconducting mechanism.

Best-possible bounds for the distribution of a sum — a problem of Kolmogorov

Abstract: Recently, in answer to a question of Kolmogorov, G.D. Makarov obtained best-possible bounds for the distribution function of the sumX+Y of two random variables,X andY, whose individual distribution functions,FX andFY, are fixed. We show that these bounds follow directly from an inequality which has been known for some time. The techniques we employ, which are based on copulas and their properties, yield an insightful proof of the fact that these bounds are best-possible, settle the question of equality, and are computationally manageable. Furthermore, they extend to binary operations other than addition and to higher dimensions.

Oxygen ordering and superconductivity in La(Ba2−xLax)Cu3O7+δ

Abstract: The observation of superconductivity above 90 K in the Y–Ba–Cu–O system1,2 has prompted a search for other structures that display high-temperature superconductivity. The discovery of superconductivity above 70 K in La3−xBa3+xCu6Ol4+y3 raises some questions concerning its relationship to the YBa2Cu3O7−δ structure and the origins of the high-temperature superconductivity. The original X-ray diffraction study of La3Ba3Cu6O14+y, indicated a tetragonal structure4,5, on the basis of which it has been proposed that La3−xBa3+xCu6O14+y is a new class of high-temperature superconductor which, in contrast to YBa2Cu3O7−δ, contains no Cu–O chains3. More recently, a neutron diffraction study of non-superconducting La3Ba3Cu6O14+y6has indicated that the correct structure of this compound is simply the tetragonal YBa2Cu3O7−δ structure with partial substitution of lanthanum on the barium site7–12. In the present neutron powder diffraction study of a series of La(Ba2−xLax)Cu3O7+δ compounds, we show that the superconducting phase is a similarly disordered isomorph of the orthorhom-bic YBa2Cu3O7−δ structure13–19. The region of solid solubility for this compound extends from x = 0.5 (La:Ba:Cu ratios of 3:3:6) to x≈ 0.25 and does not include the stoichiometric compound x = 0 (LaBa2Cu3O7−δ). The presence of Cu–O chains, and not just a formal copper charge state of ≥2, is vital for the occurrence of superconductivity.

A New Approach for Minimizing Power Losses and Improving Voltage Profile

Abstract: This paper presents a new method for minimizing transmission line losses and improving voltage profile in a given system by adjusting control variables, i.e., tap position of transformers and reactive power injection of VAR sources. Transmission losses are considered as a function of voltage increments. The control variables and voltage increments are related by a modified Jacobian matrix. Linear Programming (LP) is used to calculate the voltage increments which minimize the transmission losses, and the adjustments of control variables are obtained by a modified Jacobian matrix. Since this method does not need any matrix inversion, it will save computational time and memory space.

Exciton binding energy in a quantum-well wire.

Abstract: A model calculation is performed of the ground-state energy of an exciton confined to a quantum-well-wire system as a function of the well's width and depth, using a variational approach. The variational wave functions used in the calculation are taken as the product of the appropriate confining Bessel functions and a hydrogenic (exponential) function. For an infinite confining potential well, the binding energy diverges as the well width decreases, while for a finite confining potential well, the binding energy reaches some peak value and then decreases to its value in the bulk material surrounding the wire. It is also shown that the optical absorption coefficient for a quantum-well wire diverges with decreasing wire size when using an infinite-confining-well model. However, for the finite-well model the coefficient reaches a peak value with decreasing wire size, and then drops to its bulk material value.

Entropy generation in convective heat transfer and isothermal convective mass transfer

Abstract: The irreversible generation of entropy for two limiting cases of combined forced-convection heat and mass transfer in a two-dimensional channel are investigated. First, convective heat transfer in a channel with either constant heat flux or constant surface temperature boundary conditions are considered for laminar and turbulent flow. The entropy generation is minimized to yield expressions for optimum plate spacing and optimum Reynolds numbers for both boundary conditions and flow rigimes. Second, isothermal convective mass transfer in a channel is considered, assuming the diffusing substance to be an ideal gas with Lewis number equal to unity. The flow is considered to be either laminar or turbulent with boundary conditions at the channel walls of either constant concentration or constant mass flux. The analogy between heat and mass transfer is used to determine the entropy generation and the relations for optimum plate spacing and Reynolds number. The applicable range of the results for both limiting cases are then investigated by non-dimensionalizing the entropy generation equation.

Comparison of α‐amylase activities from different assay methods

Abstract: a-Amylase enzymes (1,4-a-~-glucanohydrolase, E.C.3.2.1.1) catalyze the hydrolysis of a l ,4 glucosidic linkages in polysaccharides of three or more a-I, 4-linked D-glucose units to produce maltose and larger oligosaccharides. 1 ~ 2 Since there are many different assay methods and definitions for a unit of a-amylase enzyme activity, it is almost impossible to compare enzyme activities. One reason is that most groups working with a-amylase developed their own enzyme assay systems, each with its own unit of a ~ t i v i t y . ~ The objective of this communication is to provide a simple relationship among a-amylase activities, which allows comparison of the enzyme activities in the literature. Even though the assay methods and definitions of an enzyme unit are different, enzyme activities can be correlated as a function of incubation temperature, incubation time, dilution factor, and measurement methods. This result will be useful in finding a microorganism or culture conditions which give the highest enzyme activity.

Chemical Structures of Sooting CH3CI/CH4/O2/Ar and CH4/O2/ Ar Flames

Abstract: A comparative study of the chemical structures of sooting, atmospheric-pressure, laminar, flat-flames of CH3Cl/CH4 were investigated using micro-probe sampling coupled with on-line mass spectrometry In the presence of chlorine, the earlier and more abundant formation of CO, C2H2 and C2H4 were noted The major reaction pathways responsible for the destruction of the fuels, and the formation and destruction of some of the intermediates are discussed

Psychophysiological correlates of induced headaches.

Abstract: The psychophysiological correlates of induced headaches were examined. Subjects with frequent muscle-contraction headaches, frequent migraine headaches, or infrequent headaches were exposed to a 1-h cognitive stressor while multiple electromyographic and cardiovascular measures were monitored. The stressor was associated with significant changes in all psychophysiological measures. Eleven of 16 headache subjects and 2 of 8 control subjects developed headaches. Significant associations between several psychophysiological variables and headache occurrence were found and individual differences in the association between psychophysiological variables and headaches intensity were examined. The results were consistent with the hypothesized role of cardiovascular factors in the pathophysiology of muscle-contraction headache. This was the first controlled demonstration of headaches occurring as a function of environmental stressors.

Photodynamic therapy of head and neck squamous cell carcinoma: optical dosimetry and clinical trial.

Abstract: This paper reports the retrospective comparison of a PDT dosimetry model with the current results of an ongoing clinical trial on photodynamic therapy (PDT) for head and neck squamous cell carcinoma (HNSCC). The model is based on the assumption that tumor eradication requires a minimum absorption of radiant energy by the tumor-localized porphyrins. The diffusion approximation was employed to calculate the incident light dose required to attain the minimum absorbed energy density at tumor boundaries most distant from the light source. Dosimetry tables for HNSCC were calculated with estimated tissue parameters, giving the PDT light dose for front surface exposure (FS) and illumination by interstitial cylindrical diffuser fibers (CI) in terms of the tumor dimensions. The model includes a correction for the photobleaching of the localized photosensitizer by the therapeutic light. The PDT trial was carried out on nine patients with previously untreated or recurrent early stage tumors and one patient with a recurrent advanced stage tumor. A complete response was obtained in 83% (10/12) of the sites treated. The calculated doses for FS and CI exposures vary from comparable with to three-fold lower than the actual doses for each complete response tumor site.

Vibrational control of an exothermic reaction in a CSTR: Theory and experiments

Abstract: Des oscillations forcees introduites dans des debits d'entree d'un reacteur continu agite exothermique, provoquent une modification de la courbe en S de regime permanent. Cette modification rend possible d'operer a un regime asymptotiquement stable localise au voisinage d'un regime permanent instable du reacteur aux debits d'entree stationnaires. Ainsi, des vibrations dans les debits d'entree peuvent etre considerees comme un mecanisme stabilisant ce type de reacteur

Absolute instability of the Gaussian wake profile

Abstract: Linear parallel‐flow stability theory has been used to investigate the effect of viscosity on the local absolute instability of a family of wake profiles with a Gaussian velocity distribution The type of local instability, ie, convective or absolute, is determined by the location of a branch‐point singularity with zero group velocity of the complex dispersion relation for the instability waves The effects of viscosity were found to be weak for values of the wake Reynolds number, based on the center‐line velocity defect and the wake half‐width, larger than about 400 Absolute instability occurs only for sufficiently large values of the center‐line wake defect The critical value of this parameter increases with decreasing wake Reynolds number, thereby indicating a shrinking region of absolute instability with decreasing wake Reynolds number If backflow is not allowed, absolute instability does not occur for wake Reynolds numbers smaller than about 38

Content-Driven Progressive Transmission of Grey-Scale Images

Abstract: A lossless progressive transmission method for grey-scale images which concentrates early transmission efforts on areas of greater image information content is described. The receiver does not have a priori knowledge of which image areas are to receive preferential treatment, and the preferential level of resolution is the pixel. The method makes use of simultaneous geometric and information content decompositions. The method is computationally simple with a complexity which grows linearly with the number of pixels. Compression achieved approaches that obtained by nonprogressive lossless methods, and is approximately the same as for homogeneous progressive lossless methods. Extensions of the method for progressive transmission with limited distortion and greater compression are also discussed.

Effects of surfactant adsorption—desorption kinetics and interfacialrheological properties on the rate ofdrainage of foam and emulsion films

Abstract: An analysis has been developed to describe the rate of drainage of an axi-symmetric plane-parallel foam and emulsion films associated with bubble-bubble and drop-drop coalescence phenomena. The present analysis extends the earlier work of Zapryanov et al(1983) and of others which was limited to the diffusion controlled case of the surfactant transport from the bulk phase onto the interface. Here the surfactant transport from the bulk phase to the interface is accounted for by a two step mechanism: (i) diffusion of the surfactant from the bulk to the sublayer, followed by (ii) the adsorption of the surfactant from the sublayer onto the interface. The results of a parametric study indicate that the rate of drainage of surfactant stabilized films is strongly dependent upon the surfactant adsorption-desorption kinetics, selective surfactant solubility, and interfacial Theological properties such as elasticity and interfacial viscosity. Predictions of the film drainage time are compared with our experimental d...

Comparison by 1H-nmr spectroscopy of the conformation of the 2600 dalton antifreeze glycopeptide of polar cod with that of the high molecular weight antifreeze glycoprotein.

Abstract: The antifreeze glycopeptide (AFGP-8) from polar cod, B. saida, is a 14-amino acid polypeptide having alternating glycotripeptide sequences of Ala-[Gal(β1 3)GalNAc(β1 O)]-Thr-Pro and Ala-[Gal(β1 3)GalNAc(β1 O)]-Thr-Ala, with alanyl residues at amino and carboxy terminals. Conformational studies of AFGP-8 have been carried out by 1H-nmr and empirical energy calculations to investigate the difference in its antifreeze behavior from that of the more active high-molecular weight AFGP 1-4 of P. borchgrevinki. The 1H-nmr spectra, including the resonances of the exchangeable amide protons, were assigned by two-dimensional correlated spectroscopy (COSY), one-dimensional difference decoupling, and nuclear Overhauser effect (NOE) measurements. For the four threonyl residues, the amide proton coupling constants and the small coupling constants between Hα and Hβ indicate similar conformations, despite significant chemical shift differences. The strong NOE between the α protons and the amide protons of the residue following together with large temperature coefficients of chemical shifts, indicate an extended conformation not consisting of α-helix, turns or bends. Energy computations indicate several low-energy conformations consistent with the observed coupling constants for ϕ. Among these, a left-handed helical conformation with three repeating residues per turn has been proposed, which is in accordance with the observed NOE between the methyl group of the α-GalNAc and Ala Hβs. While the observed Overhauser effects in the threonyl side chain suggest a certain amount of conformational averaging, the effect involving the acetmido methyl of α-GalNAc and Hβs of Ala indicate that it as is a major conformer. In view of the close similarity between the conformations of AFGP-8 and the more active antifreeze polymer, AFGP 1-4, we propose that the difference in their activities is due to the length of the regular repeating structure with glycosylation at every third amino acid residue, and not due to any fundamental difference in their conformations.